How to remove every character from a string except certain substrings (given as a list) in JavaScrip-CodePudding

In JavaScript, let's assume we have a string: "The quick brown fox jumps over the lazy dog"

And then we have a list of substrings, say: ["dog", "brown", "The", "jumps"]

How to filter every other character from the string, but not the substrings given as the list?

So the result in this case should be: "Thebrownjumpsdog"

The first solution that came to my mind was to use a loop, and RegExp on every iteration i.e:

const listOfSubstrings = ["dog", "brown", "The", "jumps"];
let theString = "The quick brown fox jumps over the lazy dog";

for (const substring of listOfSubstrings) {
  theString = theString.replace(new RegExp(`[^${substring}]`, "g"), "");
}

However, the if we take a closer look (or test) the code, we see and understand that there is nothing left after the loop: On every iteration everything but the current element in the list is removed. To be precise, nothing is left after the second iteration.

So, any ideas how the end result, I provided, could be achieved given the string and the list of substrings?

CodePudding user response：

You could match these substrings and join them:

let res = s.match(/(?:dog|brown|The|jumps)/g).join("");

See this JS demo at tio.run or regex demo at regex101

To build the pattern from the listOfSubstrings

let regex = new RegExp('(?:'   listOfSubstrings.join("|")   ')','g');

CodePudding user response：

You can try this:

const listOfSubstrings = ["dog", "brown", "The", "jumps"];
let theString = "The quick brown fox jumps over the lazy dog";

let result = "";

const theStringArray = theString.split(" ");

theStringArray.forEach(s => {
    if(listOfSubstrings.includes(s)){
        result  = s;
    }
})

But this might be slower if your listOfSubstrings is larger. For that, you can convert your listOfSubstrings to dictionary

const listOfSubstrings = ["dog", "brown", "The", "jumps"];
let theString = "The quick brown fox jumps over the lazy dog";

let result = "";

const theStringArray = theString.split(" ");

let substrings = {};

// converting array to dictionary
listOfSubstrings.forEach(e=>{
    substrings[e] = e;
})

theStringArray.forEach(s => {
    if(substrings[s] !== undefined){
        result  = s;
    }
})

The reason why using dictionary is that checking if the key exist, works in O(1) but array.includes works in O(n).